Thermal Expansion and Water Heaters


Thermal expansion is the term used to describe the expansion of water volume due to heating.  All water heaters, regardless of heat-source can experience thermal expansion.  In every tank-type water heater, cold water is heated as it enters the water heater tank.  The heating increases the overall water volume and pressure inside the tank and must be relieved to avoid the potentially adverse effects of thermal expansion.


Thermal Expansion


To understand the importance of a thermal expansion tank, it is first helpful to provide some background information on thermal expansion.  When water heats in a tank-type water heater from room temperature to 120 degrees Fahrenheit, it causes the water to expand.  This phenomenon is known as thermal expansion.  For example, when 40 gallons of cold water is heated to 120 degrees Fahrenheit or more it expands to 40.6 gallons.  A thermal expansion tank manages the thermal expansion, the extra .6 gallons, and the system water pressure. 


Thermal Expansion Tanks


A thermal expansion tank or expansion vessel is a small tank used to protect closed (i.e., not open to atmospheric pressure) domestic hot water systems from excessive pressure.  The modern vessel is a small container or tank divided in two by a rubber diaphragm.  One side of the thermal expansion tank attaches to a home’s water supply system that contains water. The other side of the tank, or dry side, holds air under pressure.  The tank also has a Schrader valve (car-tire type valve stem) for checking pressures and adding air when needed.


Open and Closed Domestic Hot Water Systems


At one time, residential hot water systems were “open” systems and did not have any backflow prevention capabilities.  Consequently, when the volume of heated water exceeded water heater tank capacity, it simply flowed back out to the city water main or water well.


Current plumbing regulations require the integration of backflow preventers or check valves into a residential water heating system, to prevent a home’s water from backflowing into the city water supply system and potentially contaminating it.  Residential hot water systems with check valves or backflow preventers are designated “closed” systems.  In a closed system, one without any thermal expansion devices, the expanded water has no place to go, the resulting pressure increases will cause a water heater’s temperature and pressure (T&P) relief valve to trigger, subsequently expelling the additional pressure and water volume through the valve.


The Negative Effects of Thermal Expansion


Even before the T&P relief valve is triggered (usually at 150 psi), excessive pressure levels caused by thermal expansion can exert forces that have adverse effects on your hot water system, such as:


  • Premature water heater failure
  • Damaged or collapsed center flue
  • Decreased water heater performance, efficiency, and product life
  • Damage to water heater connections and a home’s piping system
  • Decreased life of a home’s water fixtures and appliances
  • Noisy water hammer in a home’s pipes


About T&P Valves


A water heater’s temperature and pressure (T&P) relief valve is designed as a safety device, used primarily in emergency situations, to relieve pressure caused by thermal expansion.  Relying solely on the T&P valve to accommodate for thermal expansion wastes water and energy and may cause the valve to prematurely fail.  A better solution is a thermal expansion tank.


Thermal Expansion Tanks – A Simple Solution to Thermal Expansion


A thermal expansion tank is the solution for thermal expansion problems.  A thermal expansion tank is a small, pre-pressurized tank with a compressible air cushion (diaphragm) that is installed on the supply side (cold water inlet) of a water heater.  The process of thermal expansion:


  • Starts when the heating and thermal expansion of water creates excess pressure within the water heater and the excess water is forced into the expansion tank, keeping pressure levels within the normal operating range of the water heater
  • Ends when the pressure within the water heater normalizes (usually after there’s a call for hot water within a home), and the water within the expansion tank is forced back into the water heater by the diaphragm


The thermal expansion tank and this simple process eliminates the potentially detrimental effects of thermal expansion.


Why Install a Thermal Expansion Tank


The reasons to install a thermal expansion tank are that it:


  • Protects a water heater and maximizes its useful life
  • Reduces the stress on a home’s hot water system and its various components including pipes, fittings, fixtures, and appliances, and
  • May be required to meet local plumbing and safety codes



GPS Plumbing, a full-service plumbing company, provides a complete line of professional thermal expansion tank:


  • Installation
  • Repair, and
  • Replacement


services for most if not all types, styles, sizes, and brands of thermal expansion tanks.   We also provide a full set of complementary water pressure management solutions both of which are offered at honest and competitive prices. 


We correct and fix irritating and potentially dangerous condition such as:


  • The buildup of unusually high pressure in a water supply system - even when a pressure reducing valve is installed
  • Pressure surges
  • Chronic or continuous dripping of a temperature and pressure (T&P) relief valve
  • Dripping faucets
  • Leaking toilet tank ball cock fill valves
  • Other water pressure related conditions


If expansive or high-water pressure conditions exist in a home, we can:


  • Quickly diagnose the problem
  • Provide a cost-effective solution, and
  • Offer a free no obligation solution quote


The sooner the better when it comes to fixing an expansive water problem.  Fixing the problem as soon as possible minimizes any potential damage to your water heater and water supply system.


When to install a Thermal Expansion Tanks


The installation of a thermal expansion tank is recommended/required when any of the following occurs:


  • When a conventional water heater is replaced
  • When a backflow device is installed in water service line
  • During a water meter replacement
  • During new home construction
  • When a pressure reducing valve is installed on the water service line
  • When PRV is replaced on the water heater
  • Anytime thermal expansion of water creates an unsafe home condition


When to Contact GPS Plumbing


Thermal expansion of water in home’s water supply system can create several irritating and potentially dangerous conditions including:


  • Pressure surges
  • Chronic or continuous dripping of a temperature and pressure (T&P) relief valve
  • Dripping faucets
  • Leaking toilet tank ball cock fill valves


When you notice, any of these conditions call GPS Plumbing.